Lithium ion batteries are one of the great successes of modern materials electrochemistry. They have gained dominance in portable electronics applications due to their advantages in working voltage and energy density over traditional secondary nickel based batteries and lead acid batteries. Although significant progress has been made since commercialization in early 1990's, lithium ion batteries today still require significant improvement of both cathode and anode materials in capacity, rate, cost, and safety to meet demand for automotive applications such as electric vehicles and gas-electric hybrid vehicles.
Secondary lithium batteries include three types of lithium batteries that basically use similar cathode and anode materials but differ in electrolyte as follows:
(1) lithium-ion (Li-ion) batteries which use liquid, organic electrolyte;
(2) lithium polymer batteries which use polymer or gelled electrolyte; and
(3) solid lithium batteries which use only solid electrolyte, (not gelled) usually inorganic solid electrolyte or sometimes polymer electrolyte.
All commercial secondary lithium batteries have an anode with a binder and a current collector substrate. The binder is used to bind the active materials particles and any conductivity additives. In their manufacture, usually a slurry containing the active material, a binder, a conductivity additive, such as carbon black and a solvent is coated on the substrate current collector of the electrode. Drying and compression of the slurry produces the electrode.
Many other anode materials, such as high capacity Si-based materials and Sn-based materials have been intensively investigated and some of them have been commercialized on a small scale. However, challenges still remain in their cycle life, e.g. concerning capacity and performance degradation during the charge and discharge cycles.
On the other hand, in addition to the great success of traditional graphite and various natural and synthesized carbon-based anode materials, nano carbon materials have attracted also much interest. Nano carbon materials, including carbon nanotubes and fullerene have already been investigated to increase the specific capacities of anodes for secondary lithium batteries. Graphene is a new class and two-dimensional (one-atom-thickness) carbon allotrope arranged in a hexagonal lattice with very strong sp2-hybridized bonds different from sphere-like fullerene and one-dimensional carbon nanotubes. Graphene has attracted great interest in both fundamental science and applied research since the isolation of single graphene sheets via mechanical exfoliation in 2004. Graphene has various remarkable properties, for example, an ultra-high surface area (2630 m2g−1), high electrical conductivity (resistivity: 10−6 Ωcm) and high chemical stability that are superior to those of carbon nanotubes (CNTs) and graphite1,2. Recent work has shown that graphene as anode of lithium ion batteries has higher capacity over commercial graphite. While many efforts have focused on improvement of initial capacity of graphene anodes, little attention has been paid to their long term cycling stability. In fact, capacity and performance degradation during cycling remains an issue for these electrodes.
Doping of heteroatom into carbon structures can tailor both chemical and electronic nature. Nitrogen doping was reported to enhance Li+ intercalation/de-intercalation in carbon nanotubes3 and increase electrochemical capacity of nitrogen-containing polymeric carbon4. However, it is unknown whether or not nitrogen doped graphene can be used as anode of secondary lithium batteries and the impact of nitrogen doping on the electrochemical performance of graphene as an anode material.
Furthermore, there is a significant need, however, for providing anode materials for use in secondary lithium batteries and secondary lithium batteries having long term improved stability for use in secondary lithium batteries without performance degradation.